A phylogenomic inventory of meiotic genes:: Evidence for sex in Giardia and an early eukaryotic origin of meiosis

Sexual reproduction in eukaryotes is accomplished by meiosis, a complex and specialized process of cell division that results in haploid cells (e.g., gametes). The stereotypical reductive division in meiosis is a major evolutionary innovation in eukaryotic cells [1], and delineating its history is key to understanding the evolution of sex [2]. Meiosis arose early in eukaryotic evolution, but when and how meiosis arose and whether all eukaryotes have meiosis remain open questions [3]. The known phylogenetic distribution of meiosis comprises plants, animals, fungi, and numerous protists [4]. Diplomonads including Giardia intestinalis (syn. G. lamblia) are not known to have a sexual cycle [5]; these protists may be an early-diverging lineage [6] and could represent a premeiotic stage in eukaryotic evolution. We surveyed the ongoing G. intestinalis genome project data [7] and have identified, verified, and analyzed a core set of putative meiotic genes-including five meiosis-specific genes-that are widely present among sexual eukaryotes. The presence of these genes indicates that: (1) Giardia is capable of meiosis and, thus, sexual reproduction, (2) the evolution of meiosis occurred early, in eukaryotic evolution, and (3) the conserved meiotic machinery comprises a large set of genes that encode a variety of component proteins, including those involved in meiotic recombination.